Problems with Digitiser Drift on a Palm TX

Posted on January 20, 2009 in Palm. - Comments [3]

Recently, my trusty TX has started to exhibit a rather annoying problem with the Digitiser. Despite using the Palm preferences to calibrate the touch-screen, the bottom of the screen no longer responds to the stylus. Annoyingly this means that the button bar below the Graffiti input area is almost inaccessible.

Here’s what I get when I try and draw some lines to the very bottom of the screen and along the bottom edge.

You can see that he lines stop a little above the bottom (left in this image) edge.

The problem is not just limited to the inaccessibility of the button bar. As you move down the screen the calibration is increasingly inaccurate.

A quick search reveals that “digitiser drift” is a relatively common problem with the Palm TX, especially as the digitiser becomes worn in use. Other symptoms include dead-spots and generally poor responsiveness.

Possible Solutions

There’s a number of things you can try and your mileage is likely to vary depending on the state of your device.

Hard Reset

Try doing a hard reset. This is suggested on Palm support site: How to correct handhelds not responding correctly to screen input/Stylus strokes).

Make sure you have a full backup of your device before trying this! You may want to check out NVBackup which can be used to backup your palm to a memory card.

It’s a good idea to test your Palm before restoring any applications. That way you can be sure that your problems aren’t being caused by another application running on the background.

PowerDiGi

The TX’s own digitiser calibration tool expects the digitiser to operate with a consistent linear resistance (see below). This may not be the case. PowerDiGi allows you to use more reference points (up to 70!) to map the fluctuations in the digitiser. This

PowerDiGi is available from Palm Powerups.

There have been reports on various forums that running PowerDigi on the TX causes the a lag in text input while using applications such as DocumentsToGo and VersaMail. This is not something that I noticed when trying PowerDiGi 2.1.0.2

PowerDiGi won’t help with dead-spots.

New Digitiser

I guess the good news is that it’s possible to replace the faulty digitiser. Obviously if your TX is still under warranty then you should return it to Palm for a repair (replacement with a re-conditioned unit). That said, many people have reported better performance from a replacement glass digitiser then with the original plastic one. (I would strongly recommend a decent case for your palm if you decide to install a glass digitiser.) Note that the bottom layer of the digitiser is glass – the top layer is plastic regardless of the type of digitiser, and just as susceptible to scratches.

Replacement parts, including digitisers are available from the following websites:

• PDA Parts (New Glass Digitiser: $39.99)
• Used PDA Parts (New Glass Digitiser: $35)

The following video might give your a feel for what you’re letting yourself in for if you decide to replace the digitiser yourself:

After I saw this video I decided that it would probably be better to find someone to do the repair for me!

Replacement Palm TX

Ultimately, if none of the above options are satisfactory then you’re left with little choice than to get a replacement TX. The price of which is still reasonably high. This is mainly because of the lack of a replacement product – Palm have stated the TX is the last PDA that they will produce and that their focus now is on SmartPhones. Palm are advertising the TX at £204 (You’ll be hard pushed to find it mentioned on the US site!)

Of course you can always look for a second-hand bargain on the likes of EBay. The only drawback with this is that you’re likely to be buying an old used unit where the state and life expectancy of the digitiser is unknown.

Why has this happened?

The Palm TX uses a ‘resistive’ digitiser. Resistive touch-screen panels are generally the most affordable technology but offer reduced clarity and the layer can be damaged by sharp objects.

The important parts of a ‘resistive’ digitiser are two thin electrically conductive and resistive layers separated by ta thin space. When the stylus is used on the digitiser the layers are pressed together. This causes a change in the electrical current and by measuring the resistance, the x-y co-ordinates can be established. A plastic digitiser as supplied in the Palm TX uses lots of small insulating bumps to keep the two layers separate. A glass digitiser uses gel to keep the layers apart.

In order for the digitiser to work accurately it is important that the electrical resistance varies consistently across the X and Y distance of the display. If this is true then the digitiser can be calibrated just by knowing the location of a small number of points. (The Palm touch-screen calibration uses just three.)

Of course, the accuracy of the digitiser will be compromised if the electrical conductivity of the digitiser is not perfectly linear (the change in resistance is proportional, to the distance you move horizontally and vertically across the membrane).

How can software such as PowerDiGi help?

PowerDiGi can help when the digitiser is returning non-linear results. It does this by sampling the resistance are a number of known locations and then mathematically compensates for this. As long as the frequency of sampled location is greater than the changes in the resistance fluctuations then PowerDiGi should be able to do a reasonable job maintaining the accuracy of the digitiser.

However, having to calculate the true location every time the digitiser is touched increases the overhead on the processor. I suspect that this could be the cause of the lag that some people have encountered and I would also guess that there would be a decrease in battery life, especially if you’re using the digitiser a lot (such as panning around web pages in Blazer!)

As the digitiser degrades even this extended calibration will become less and less accurate. You can test the accuracy of the compensated position using the PowerDiGi test button. As you move the stylus around the display you can easily see how the cross-hairs drift away from the real position.

What am I going to do?

In order to get a reasonable performance from my TX, I’ve installed PowerDiGi and have had to use the full 70 samples to map the digitiser. Even then, the accuracy at the bottom of the screen is a little off but at least I can still click on the buttons and get to the bottom of the display.

It seems that if the digitiser degrades any further it will be necessary to replace it. I’m just waiting for a quote for the work and worrying about how I’m going to cope without it for a while. If I’m going to have the work done, then I’ll probably get the battery replaced with a long-life version too as the battery’s performance over time has diminished.

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